People keep telling me I'm way too soft, and others take advantage of me a lot. Is it because I'm so polite?

Yea I think you're being too polite and sometimes you have to be more aggressive. This is what my manager told me at work. Check my question about my co-worker to further understand it. Similar thing happened to me when a guy asked me for a change and gave him way more than what he was asking just to find out he spent it on beer!